It has long been known to control motors, and particularly DC motors, by varying the applied voltage. Various approaches to such voltage variation have been employed, including variable resistances, SCR controls, and numerous other configurations. Other approaches have been to supply with motor with sawtooth, square or other variable waveforms, such that the total power supplied to the motor can be controlled either by the ramp of the sawtooth, the duty cycle of the square wave, or other variations.
In those instances where square waves have been employed, it has been known to employ an inverter to convert a DC control signal to a variable duty cycle pulse train. Such inverters typically employ a bistable multivibrator to generate a pulse train, which creates difficulties in that the maximum duty cycle of such circuits is less than 100 percent, and also tends to drift. In addition, most if not all of such inverters have employed an output stage which operates at least a portion of the time in saturation to minimize voltage across the output transistors at high currents. Such devices have typically also employed emitter resistors to indicate the condition of the output transistors, which can create difficulties when paralleled devices are not perfectly matched or become slightly mismatched through thermal cycling or other effects.
In addition, while inverters operating in saturation are acceptable for relatively low speed operation, the maximum frequency of operation is limited by the time required for the charge stored in the saturated devices to be removed. While such low speed operation is acceptable for certain applications, there are many applications where relatively high frequency is either desirable or required. For example, ultrasonic operation is particularly useful where the device is to be operated around humans, since relatively low frequency operation is audible and annoying. In addition, in applications such as electric vehicles, higher frequency operation provides smoother performance.